A charged particle detector is an indispensable part of charged particle (ion or electron beam) instruments, such as a scanning electron microscope (SEM). In a SEM, an electron beam emanated from an electron source is focused into a fine probe over a specimen surface and scanned by a deflection unit in a raster fashion; and signal electrons released from the specimen, including secondary electrons and back scattered electrons, are collected by charged particle detectors and the signal intensity is converted into the gray level of an image pixel corresponding to the location of the electron probe on the specimen surface. Scanning of the electron probe will then form a gray level mapping thus producing an image of the specimen surface. A low voltage SEM, in which an incident electron beam has the energy of 3 keV or less, is known to be particularly effective in evaluating topographic features of the specimen surface due to the dominance of secondary electrons in the signal electrons. Secondary electrons are originated within a shallow depth from the specimen surface; their yield and trajectory are influenced by the surface topography and thus carry the topographic information. (A detailed description on all aspects of low voltage SEM operation can be found in “Image Formation in Low Voltage Scanning Electron Microscopy” by L. Reimer, SPIE Optical Engineering Press, 1993.)
The most common detectors used in SEM are of the scintillator-photomultiplier tube (PMT) combination type (such as an Everhart-Thornley detector), the semiconductor type, and the microchannel plate type. The scintillator-PMT type, due to their high gain and low noise properties, is more frequently used in high resolution SEMs in which the beam current is low. Traditionally this type of detector is made of a light guide rod, its front face coated with a light-generating scintillator, coupled to a photomultiplier tube. A common arrangement is to position one or a multiple of these units below the final focusing objective lens, surrounding the impact point of the primary electron beam, with the front face covered with a positively biased grid to attract secondary electrons emitted from the specimen in what amounts to a side detection scheme (see, for example, U.S. Pat. No. 4,818,874). This is schematically illustrated with detector 108 in FIG. 1, where the basic structure of a SEM is shown and the side detector 108 is inserted between the objective lens 103 and the specimen 104. Recently, increased demand on low voltage SEMs of higher resolution has prompted more widespread use of SEMs with an immersion type of objective lens for its ability to provide finer electron probes due to smaller electron optical aberrations. In a SEM with an immersion type of objective lens, the specimen is immersed in the strong magnetic focusing field of the objective lens, over which an electrostatic extraction field is also typically superimposed. While the main purpose is to focus the primary electron beam, the magnetic field also confines the secondary electron trajectories close to the central optical axis, with the electrostatic field acting to pull the electrons away from the specimen into the center bore of the objective lens. In this case, side detectors can no longer receive any secondary electrons, and in-lens detectors must be used instead. For the best secondary electron collection efficiency, on-axis annular type in-lens detector is preferred. An example is shown with detector 109 in FIG. 1, which is positioned on the optical axis 110, with a center hole in the middle for the primary charged particle beam 111 to pass through.
It is well known in the art that segmented on-axis annular detectors can be used for enhancing topographic features on the specimen surface without specimen tilt. Such a detector is divided into equal halves or quarters, with signal output from each segment processed and displayed separately. An example is shown in FIG. 2. Secondary electrons 205B emanated from side surface 204L of a surface feature 208 strike only detector half 206B, while those 205A from side surface 204R strike only detector half 206A, after crossing the optical axis under the focusing effect of the magnetic immersion field from the objective lens 203. When the signals from each half are added together as in a non-segmented detector case, both edges of the feature will show up bright in the image (as shown in the line profile 2A) as more secondary electrons are emitted along the side wall region, making it difficult to discern whether the feature 208 is a protrusion or a depression. However, when the signal from each half is displayed separately, a shadow effect is created with one edge of the surface feature 208 showing up bright while the other showing up dark, as shown in the line profile 2B from detector half 206A and the line profile 2C from detector half 206B, creating a three-dimensional impression which helps to determine that the feature 208 is a protrusion.
Segmented on-axis annual detectors have been most commonly constructed with semiconductor or microchannel plate type of detectors due to their planar nature, which can be easily made into a segmented round plate. Examples include a back-lens detector where the detector is placed upstream in the back of the objective lens, and a front lens Robinson detector where the detector is attached to the lower face of the objective lens pole piece facing the specimen. Processing of the signals from each segment of the detector, including addition or subtraction, can provide large quantity of information of the specimen including topographic and material information. However, segmented on-axis annual detectors are rarely made with Scintillator-PMT type of detectors doe to the difficulty in light-guide design because of their non-planar nature, even though this type of detector can be the superior choice in many cases. In those cases, either a single detector is used on axis or multiple detectors are arranged separately around the center optical axis, often working in conjunction with a reflection plate which generates its own secondary electrons to be collected by the detectors, when the plate is struck by the secondary and back-scattered electrons coming from the specimen (see, for example, U.S. Pat. No. 6,617,579). These arrangements are schematically illustrated in FIG. 1, with multiple in-lens detectors 106A and 106B positioned off axis in conjunction with a reflection plate 107, or a single Scintillator-PMT detector 105 positioned on axis. For the multiple detector arrangement the construction is complicated and the signal collection efficiency is not the best due to the spatial separation of the individual detectors, while for the single on-axis detector case it is difficult to achieve a uniform signal collection due to the non-rotational-symmetric nature of the light-guide tube.
Accordingly, there is a need in the art for innovative designs for charged particle detectors, so that the high efficiency space saving segmented on-axis annular configuration or its equivalent can be realized for Scintillator-PMT type of detectors.